Deferred Shading

About 2 weeks ago Azaezel, Timmy and myself set out to bring Physical Based Shading to Torque. One of the biggest hurdles we ran into is the lighting information we need is not available at the stages we need it. This is due to Torque's prepass lighting (aka deferred lighting).

How does deferred lighting work?

Everything is rendered twice. Once to calculate lighting, and then everything is rendered again + the previously obtained lighting information for your final lit scene. This is why a 30k poly model loaded into Torque will show 60k.

This may sound very expensive but it's made in a way that the light prepass is as cheap as possible. It's by no means a bad method of doing things, nor was it a poor choice at the time. In fact, most engines around that time frame used deferred lighting. It also has the advantage of running better on older hardware.

How does deferred shading work?

Everything is rendered once, but it's done in a rather clever way. When rendering, a pixel shader is used to write all the data needed to different buffers. One for depth/normals, one for lighting (specular info, etc), and one for color. Then at the end a shader is used to combine all the information from the different buffers into a final picture.

This reduces rendering to once, but puts more stress on the graphics card and uses more video memory. This was not necessarily considered the better choice until recent years when graphics hardware has gotten better.

Why make the switch?

I hate to say "because the other guys are doing it" but if you look into it the big players that were using deferred lighting have made the change to deferred shading ( see: UE4 ). This will allow us to easily implement different lighting models like cook-torrance.

It also has the advantage of allowing some additional shaders and postfx that we couldn't really do before. Take for instance Lukas' SSGI shader. One big issue he ran into is the lack of an available albedo (or color) buffer to read just color data from. This is available with deferred shading.

What's the status?

We've been working on the conversion for about a week now and everything seems to be working quite well. There's a few missing features, and a handful of broken things but it's functional and working. Performance is about the same as before, which we consider to be a good thing since we haven't attempted any kind of optimization yet.

Conclusion

It's a pretty big change and will have a huge impact on materials, material shaders, and lighting. While we plan to continue working on this regardless for personal use, we would like to know if this is something the community is interested seeing make it's way into the main repo. This wouldn't be a 3.6 or 3.7 change, but further down the line (perhaps 4.0?).

Links

Link to the development repo:
github.com/andr3wmac/Torque3D/tree/deferred_shading

A guy's benchmarking of deferred lighting vs shading:
frictionalgames.blogspot.ca/2010/10/pre-pass-lighting-redux.html

An article that evaluates Deferred Lighting VS Deferred Shading:
gameangst.com/?p=141